Round To 5
August 13, 2019
Every morning when I drive to work the highway signs display commute information: 7 minutes to Manchester Road, 11 minutes to Olive Blvd, speed ahead 55 mph. That’s little comfort when I’m sitting still on the highway. The “speed ahead” calculation is always rounded to the nearest 5 mph; the Department of Transportation takes a bunch of speed readings from sensors buried in the pavement and averages them.
Your task is to take a list of speed readings and average them to the nearest 5 mph. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.
Programming Praxis 
Here is a slightly more general solution in standard R7RS Scheme (and
a couple of SRFI helpers only for testing). It allows rounding to
non-integer values too, e.g., the nearest multiple of 25, as the demo
illustrates.
(import (scheme base) (scheme write) (only (srfi 1) list-tabulate) (only (srfi 27) random-integer)) (define (rounded-average delta nums) (* delta (round (/ (apply + nums) (length nums) delta)))) ;; only demo/testing below (define (demo/nums nums) (display (rounded-average 5 nums)) (display (rounded-average 0.25 nums)) (display (inexact (/ (apply + nums) (length nums)))) (newline)) (define (demo1) (demo/nums (list-tabulate 20 (lambda (_) (random-integer 100))))) (define (call-times n proc) (let rec ((i 0) (r #f)) (if (>= i n) r (rec (+ i 1) (proc i))))) (define (demo) (call-times 10 (lambda (_) (demo1)))) (demo)Output:
Klong version: 10 iterations
Code: rnd5::{5*_(x+2.5)%5}; test::{[l]; l::[]; {~x>10}{l::l,40+_19*.rn(); x+1}:~1; .p("avg = ",avg::(+/l)%#l); .p("avg5 = ",rnd5(avg))}; {x<10}{test(); .p(""); x+1}:~0; "" Run: [avg = 47.4] [avg5 = 45] [avg = 46.9] [avg5 = 45] [avg = 49.6] [avg5 = 50] [avg = 47.1] [avg5 = 45] [avg = 46.7] [avg5 = 45] [avg = 49.3] [avg5 = 50] [avg = 45.0] [avg5 = 45] [avg = 48.4] [avg5 = 50] [avg = 48.0] [avg5 = 50] [avg = 50.1] [avg5 = 50] ""Here’s a solution in Python.
def round5(l): avg = round(sum(l) / len(l)) offset = [0, -1, -2, 2, 1] avg += offset[avg % 5] return avg l = [51, 68, 45, 41, 44, 68, 80, 52, 45] print(round5(l))Output:
Rust version
fn main() { let x = vec!(12.1,25.2,27.4,32.1,21.5,1.7,5.6,88.,42.0,19.12,); let avg = x.iter().sum::<f64>()/x.len() as f64; let answer = (avg/5.0 + 0.5) as usize * 5; println!("{}", answer); }